Electric power is mostly supplied by power grids of a power company through a transmission network of a bulk power grid. The frequency of a power grid results from the balance between demand (load) and supply (generated output). Thus, the power company matches the supply and demand on a moment-by-moment basis by controlling load fluctuation of at most several tens of seconds by a governor-free power generator, load fluctuation of at most 20 minutes by a power plant with a frequency control (AFC) function, and a load fluctuation of the order of several hours by scheduled increases and decreases in the output of a steam turbine power plant. A power generator in the grid includes a group of synchronous power generators and has a control characteristic called drooping. The control adjusts the output so as to increase the rotation of the power generators with decreased grid frequency, while reducing the rotation of those with increased grid frequency. Thus, owing to the drooping characteristic, all the power generators cooperate with one another in maintaining constant frequency. Furthermore, these synchronous power generators are rotating machines with a large inertia force and thus have the ability to stabilize the frequency in the area without being affected by slight fluctuations in grid frequency. This is expressed by the phrase “synchronizing power of the power generators”.
In Japan, the power supply is separated into two grids, a 50-Hz grid for the eastern region and a 60-Hz grid for the western region, therefore demand control for frequency stabilization is performed independently in the respective regions. This control allows the frequency to be synchronized to a single value in all locations within each of the two regions.
On the other hand, in recent years, renewable energy power sources such as wind power generation, solar power generation, and biofuel power generation have been introduced at an accelerated rate as a means for dealing with the problem of global warming and as a result of the rapid rise in fossil fuel costs.
The Japanese government has established the goal of introducing 28,000,000 kW of solar power generation by 2020 and 53,000,000 kW of solar power generation by 2040, however on the other hand, as reported in “Report of Workshop on Low Carbon Power Supply Systems” in July, 2009 the current power grid, if not improved, could support only about 13,000,000 kW of solar power generation.
This limitation is because renewable energy power sources such as solar power generation and wind power generation cannot provide synchronizing power.
These power sources provide power through inverters which detect the frequency of the grid and feed current synchronously by following other power sources on the grid. The power sources thus have the tendency of impairing the frequency stability of the system by magnifying normal power fluctuations. Thus, the introduction of large amounts of renewable power that fluctuates severely impairs the synchronization capability of the existing grid. This may lead to a catastrophe such as cascading massive power failure.
To solve this problem and introduce a large amount of renewable power, it is necessary to design a new power grid not supported by conventional concepts and to present a procedure for migrating the current grid to the new system without critical difficulties. However, the conventional art includes no document or report regarding such a system or procedure.
To deal with problems occurring when a large amount of unstable power is connected to a synchronous grid, several conventional techniques as described below have been proposed. The techniques are roughly classified into the following three methods.
A first conventional method is to enhance bulk power grids. That is, fluctuations in renewable energy power generation is remediated by enhancing high-voltage interconnection lines, installing back-to-back (BTB) loop controllers, increasing the capacity of frequency converter stations, increasing the capacity of DC interconnection lines between Hokkaido and Honshu, Japan, and increasing the number of gas turbine power generation facilities and variable speed hydroelectric power generation facilities as backup power sources. This method relates to Patent Literature 1 and 2 listed below.
A second conventional method is suppression of output from distributed power sources and reducing load (demand). For output suppression, every effort has been made to require power companies of solar power generation or wind power generation to provide a circuit that suppresses the output in response to a signal from the power company. This method relates to Patent Literature 3 and 4 listed below.
A third conventional method is to interchange power among a plurality of power grids or between a bulk system and power grids. This method uses a power interchange device to connect together a plurality of power grids into which a large amount of unstable power such as renewable energy is introduced so that power can be interchanged among the power grids. This method relates to Patent Literature 5, 6, 7, and 8 listed below.
Furthermore, a combination of power and communication is described in Patent Literature 9 listed below.